The present invention relates to ion sources such as those used in, e.g., mass spectrometers. In particular, the present invention relates to an ion source which operates using only a single potential.
Mass spectrometers are known in the art, and may be used to measure the presence of a selected gas in a system. A central component of a typical mass spectrometer is the ion source. Gas entering the mass spectrometer flows into the ion source. Electrons, produced typically by a hot filament, enter an ion chamber and collide with the gas molecules. This creates an environment within the chamber where ions are quantitatively proportional to the pressure in the ion chamber. Ions are withdrawn from the ionization chamber through an exit hole or slit under the influence of an electrostatic field created by a voltage potential applied at a withdrawal electrode. The ions are further guided by one or more focus plates which also produce a field created by further voltage potentials. The various voltage potentials creating the ion beam and the focus fields are chosen to ensure that a straight ribbon of ions exits from the chamber.
The ions from the chamber typically enter a magnetic field which deflects ions in proportion to their mass-to-charge ratio. In magnetic bending types of helium mass spectrometer leak detection systems, the magnetic field is typically adjusted so hydrogen ions are deflected 135.degree., helium ions 90.degree., and all heavier species less than 90.degree.. An ion collector is placed at 90.degree. to collect the target particles, i.e., helium ions. All other ions are deflected away from the collector. The collector current is then measured by an amplifier for evaluation.
In these previous systems, the ion source required the application of a number of voltages to create necessary electron trajectories and to withdraw and collimate a stream of ions for delivery through the magnetic field. Most previous systems required at least four or five different voltage sources to accomplish this. This is undesirable for several reasons. When these voltages are varied to obtain the desired helium ion beam current and shape they tend to interact and thus require a series of iterative adjustments which makes an automatic tuning more difficult. The iterations required makes the adjustment procedure rather lengthy. Further, construction, design, and coupling of ion sources requiring several potentials is difficult. With increased complexity comes reduced reliability and increased cost.
Another disadvantage of existing ion sources is that they have a limited useful life. The life of the source is only as good as the life of the electron emitting filament used in the system. Although certain existing systems use redundant filaments (i.e., a spare is typically placed opposite or beside the primary filament for use when the primary expires), the life of the ion source is still limited. Once both filaments have expired, the ion source is rendered useless until the source can be retrofitted with new filament(s).
Accordingly, an ion source for a mass spectrometer leak detection system is needed which is easily tuned, simple in design, low in cost, and long in life. Further, it would be desirable to provide an ion source which supports automatic tuning.